1. Field of the Disclosure
The present disclosure relates to flyback power converter; in particular, to a flyback power converter which is able to perform a synchronous rectification under either a continuous conduction mode or a discontinuous conduction mode.
2. Description of Related Art
In recent years, there is a boost in developing a switching power supply and the switching power supply has been played an extremely important role in computers and other electronic products. In comparison to a traditional power supply, the switching power supply has advantages such as higher stability, simplicity, and efficiency. Among which, a flyback power converter has been broadly used due to a lower cost and a simplified circuit design.
A common method of controlling the flyback power converter in synchronous rectification is to use a synchronous rectification control chip to detect a drain-to-source voltage of a synchronous rectification switch. When a primary side switch is cut off, the secondary-side body diode of the synchronous rectification switch is forced to be switched on to provide a current path allowing a magnetizing inductance to release energy. During the time, an absolute value of the drain-to-source voltage of the synchronous rectification switch is higher than a switched-on threshold conductive voltage predetermined by the synchronous rectification control chip, and thus the synchronous rectification switch is enabled by the synchronous rectification control chip. With the magnetizing inductance gradually releasing power to a load, the absolute value of the drain-to-source voltage decreases, and once the absolute value be lower than a cut-off threshold voltage predetermined by the synchronous rectification control chip, the synchronous rectification switch is disabled by the synchronous rectification control chip.
However, the method is only ensured that the synchronous rectification switch is successfully disabled before a primary side switch switched on in a next switching period under a discontinuous conduction mode. If it is under a continuous conduction mode, a continuous magnetizing inductance current leads the absolute value of the drain-to-source voltage easily higher than the cut-off threshold voltage predetermined by the synchronous rectification control chip within a switching period, and it will be until the primary side switch to be switched on in the next switching period, the magnetizing inductance transformed from releasing energy to storing energy, the secondary side of a transformer senses a process that a corresponding voltage transformed from a negative voltage to a positive voltage, and the absolute value of the drain-to-source voltage is lower than the cut-off threshold voltage predetermined by the synchronous rectification control chip, and finally the drain-source voltage of the synchronous rectification switch is disabled by the synchronous rectification control chip. Although the method is finally able to disable the synchronous rectification switch, there is a short moment when both the primary side switch and the synchronous rectification switch are switched on before the synchronous rectification switch is disabled, and thereby a problem of shoot-through is caused. As a result, a current stress of the synchronous rectification switch is increased, and a voltage surge of the drain-to-source voltage of the synchronous rectification switch also increases when the synchronous rectification switch is switched off, which increases the voltage stress of the synchronous rectification switch along with a switching consumption; therefore, an overall switching efficiency is lowered, a more cost in high voltage and current stresses of the synchronous rectification switch is caused.